It is commonplace in the packaging of a wide variety of materials ranging from pharmaceutical products to instant coffee that a closure is provided in the form of a seal connected to the neck of a container and a screw cap covering and protecting the seal which provides a re-closable cap after the seal has been removed to gain access to the container. Often the closure is such that the underside of the seal has a heat sensitive adhesive coating or a meltable plastics layer covered by a metal foil. The metal foil can provide the substrate of the seal or may include a separate substrate formed from plastics material or paper. The seal is then placed against the neck of a container and sandwiched against it by the applied screw cap. An induction heating step then heats the metal foil and in turn activates the heat sensitive adhesive layer or melts the plastic layer so that on cooling, the seal bonds to the neck of the container. A difficultly often encountered by eventual users is removal of such seals from the container. Attempts have thus been made to include a tab extending sideways from the neck of the container so that the consumer can grip this to facilitate removal of the seal.
Single component systems have been developed wherein the primary seal and secondary liner are produced as a laminate disk which is attached within the cap. The commercially available Quadraseal (®) system is an example of a single component system. The bottom layer of the seal portion is formed of glassine which is attached to a foil layer by means of a wax based adhesvive. Upon removal of a cap into which the system is fitted, the seal portion remains adhered to the neck of the container. It can be removed by puncturing the seal and tearing it off in pieces. Due to failure of the glassine fibres, there will be traces of fibres remaining adhered around the circumference of the neck.
A further example is DE9108868 in which the primary seal and secondary liner are adhered by means of wax. On heating of the metal foil in the primary seal portion the wax melts and is absorbed into the secondary liner whereby the primary seal and secondary liner substantially separate from each other. On opening the primary seal remains adhered to the container and the secondary liner remains in the cap. This system includes a tab which is formed by only adhering the top layer of primary seal to the remainder of the seal across part of the diameter of the seal. One problem with this system is that there are residues of wax material left on the surface of the liner such that when the container is resealed, some of the contents of the container may stick to the liner surface or may be released into the container itself. In the packaging of food and pharmaceuticals, such a wax residue is undesirable, Attempts have therefore focussed on the use of a plastics based means for adhering the primary seal to the secondary liner as a so-called release layer to avoid the use of wax. It is also very difficult to ensure that during attachment of the seal to the container to be sealed, the heat produced does not cause the delaminated portion which forms the tab to adhere to the remainder of the primary seal portion.
A further example of a system including a tab structure is the “Top Tab” structure disclosed is U.S. Pat. No. 4,961,986. This structure includes a multilayer primary seal, one layer of which is partly de-laminated to provide a lifting tab lying wholly within the circumference of the container neck in use. This system is essentially a system comprising a primary seal and secondary liner whereby the secondary liner has been adhered to a cap. The primary seal is then pushed into the cap and held in place by engagement with the screw thread within the cap. In this system, the tabs are adhered to the primary seal by applying an adhesive layer in strips and then adhering the top layer with the result that it is only stuck in particular portions where the adhesive was present. Such a method is time consuming as it involves the gradual layer-by-layer construction of the primary seal including the tab and involves the use of a specifically formed roller to apply the adhesive in strips.
A further tab system is the successful “Lift “n” Peel” (®) commercial system. This can be comprised of a primary seal and a secondary liner or just a primary seal and is usually just a primary seal. The primary seal includes a thick layer of foam such that the seal can be a product in its own right without requiring a secondary liner to give it stability. The thickness of the foam allows for discrepancies in the container or cap size. However, the foam is expensive and the resulting primary seal is difficult to manipulate. A further disadvantage associated with including foam is that it is not a printable substrate, Nowadays, consumers rely heavily on the labelling of goods and it is desirable to be able to provide a seal that is printed to indicate the manufacturer. It is usual that the printing is done on the foil layer included within the heat induction layers. This will only be seen where the layers on top of the metal foil are transparent. The “Lift “n” Peel” system does not present this opportunity due to the opaque nature of the foam. The top layer of the Lift “n” Peel system is a particular polyethylene terephthalate/ethylene vinyl acetate (PET/EVA) structure. The presence of EVA is necessary to ensure adhesion to both the PET tab and the foam. If the EVA were to be printed then this would interfere with the adhesion properties it exhibits. Hence reverse i.e. printing the underside of the top layer of the “Lift “n” Peel” system is not a viable option. The PET/EVA structure used is an expensive material and additionally requires the use of heated rollers to melt the EVA component of the structure to achieve adhesion. The lower the amount of EVA used, the greater the size of the heated roll required. Thus to use a material of low cost which contains minimal EVA, the size of the roll required becomes prohibitively large and expensive.
In WO 97/02997, a method for including a tab in a primary seal laminate is disclosed. In this case three feeds are passed to a chill roll in contact with a nip roll. The first feed which will form the top layer of the primary seal laminate, is EVOH coated with polypropylene. The second feed is a deadening member and the third feed is extruded tabstock e.g. polypropylene. All three feeds come into contact at the point where the chill roll and nip roll are in contact. The chill roll quenches the extruded tabstock maximising its amorphous properties and causes lamination of all three layers.